[ELF] Set SHF_INFO_LINK for .rel[a].plt and .rel[a].dyn

[MaskRay]

Summary

The ELF spec says

If the sh_flags field for this section header includes the attribute SHF_INFO_LINK, then this member represents a section header table index.

Set SHF_INFO_LINK so that binary manipulation tools know that sh_info is
a section header table index instead of (the number of local symbols in the case of SHT_SYMTAB/SHT_DYNSYM).
We have already added SHF_INFO_LINK for --emit-relocs retained SHT_REL[A].

For example, we can teach llvm-objcopy to preserve the section index of the sh_info referenced section if
SHF_INFO_LINK is set. (GNU objcopy recognizes .rel[a].plt and updates
sh_info even if SHF_INFO_LINK is not set).

[grimar]

Looks reasonable. Probably worth waiting to see if others have objections.

[MaskRay]

@psmith Does this also look good to you? (It is to be consistent with GNU ld and LLD's own --emit-relocs SHT_REL[A])

[psmith]

LGTM too. Apologies for the delay. This looks like it is consistent with the ELF specification.

lld/ELF/SyntheticSections.cpp

void RelocationBaseSection::finalizeContents() {
   // When linking glibc statically, .rel{,a}.plt contains R_*_IRELATIVE
   // relocations due to IFUNC (e.g. strcpy). sh_link will be set to 0 in that
   // case.
   if (symTab && symTab->getParent())
     getParent()->link = symTab->getParent()->sectionIndex;
   else
     getParent()->link = 0;
 
-  if (in.relaPlt == this)
+  if (in.relaPlt == this) {
+    getParent()->flags |= ELF::SHF_INFO_LINK;
     getParent()->info = in.gotPlt->getParent()->sectionIndex;
-  if (in.relaIplt == this)
+  }
+  if (in.relaIplt == this) {
+    getParent()->flags |= ELF::SHF_INFO_LINK;
     getParent()->info = in.igotPlt->getParent()->sectionIndex;
   }
+}
 
 RelrBaseSection::RelrBaseSection()
     : SyntheticSection(SHF_ALLOC,
                        config->useAndroidRelrTags ? SHT_ANDROID_RELR : SHT_RELR,
                        config->wordsize, ".relr.dyn") {}
 
 template <class ELFT>
 static void encodeDynamicReloc(SymbolTableBaseSection *symTab,

lld/test/ELF/aarch64-gnu-ifunc.s

 // REQUIRES: aarch64
 // RUN: llvm-mc -filetype=obj -triple=aarch64-none-linux-gnu %s -o %t.o
 // RUN: ld.lld -static %t.o -o %tout
 // RUN: llvm-objdump -d --no-show-raw-insn %tout | FileCheck %s --check-prefix=DISASM
 // RUN: llvm-readobj -r --symbols --sections %tout | FileCheck %s
 
 // CHECK:      Sections [
 // CHECK:       Section {
 // CHECK:       Index: 1
 // CHECK-NEXT:  Name: .rela.dyn
 // CHECK-NEXT:  Type: SHT_RELA
 // CHECK-NEXT:  Flags [
 // CHECK-NEXT:    SHF_ALLOC
+// CHECK-NEXT:    SHF_INFO_LINK
 // CHECK-NEXT:  ]
 // CHECK-NEXT:  Address: [[RELA:.*]]
 // CHECK-NEXT:  Offset: 0x158
 // CHECK-NEXT:  Size: 48
 // CHECK-NEXT:  Link: 0
 // CHECK-NEXT:  Info: 4
 // CHECK-NEXT:  AddressAlignment: 8
 // CHECK-NEXT:  EntrySize: 24

lld/test/ELF/arm-combined-dynrel-ifunc.s

  .word indirect(got)
  .word bar2(got)
  bx lr
 
 // CHECK:     Name: .rela.dyn
 // CHECK-NEXT:     Type: SHT_REL
 // CHECK-NEXT:     Flags [
 // CHECK-NEXT:       SHF_ALLOC
+// CHECK-NEXT:       SHF_INFO_LINK
 // CHECK-NEXT:     ]
 // CHECK-NEXT:     Address:
 // CHECK-NEXT:     Offset:
 // CHECK-NEXT:     Size: 24
 
 // CHECK: 0x00000012 RELSZ                24 (bytes)
 // CHECK: 0x00000002 PLTRELSZ             8 (bytes)

lld/test/ELF/arm-gnu-ifunc.s

 
 // CHECK: Sections [
 // CHECK:   Section {
 // CHECK:        Section {
 // CHECK:          Name: .rel.dyn
 // CHECK-NEXT:     Type: SHT_REL
 // CHECK-NEXT:     Flags [
 // CHECK-NEXT:       SHF_ALLOC
+// CHECK-NEXT:       SHF_INFO_LINK
 // CHECK-NEXT:     ]
 // CHECK-NEXT:     Address: 0x100F4
 // CHECK-NEXT:     Offset: 0xF4
 // CHECK-NEXT:     Size: 16
 // CHECK-NEXT:     Link:
 // CHECK-NEXT:     Info: 4
 // CHECK:          Name: .iplt
 // CHECK-NEXT:     Type: SHT_PROGBITS

lld/test/ELF/dynamic-reloc.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 // RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/shared.s -o %t2.o
 // RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/dynamic-reloc.s -o %t3.o
 // RUN: ld.lld -shared %t2.o -soname=so -o %t2.so
 // RUN: ld.lld %t.o %t3.o %t2.so -o %t
 // RUN: llvm-readobj --dynamic-table -r -S %t | FileCheck %s
 
 // CHECK:      Index: 1
 // CHECK-NEXT: Name: .dynsym
 
 // CHECK:      Name: .rela.plt
 // CHECK-NEXT: Type: SHT_RELA
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
+// CHECK-NEXT:   SHF_INFO_LINK
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address: [[RELAADDR:.*]]
 // CHECK-NEXT: Offset:
 // CHECK-NEXT: Size: [[RELASIZE:.*]]
 // CHECK-NEXT: Link: 1
 // CHECK-NEXT: Info: 9
 // CHECK-NEXT: AddressAlignment: 8
 // CHECK-NEXT: EntrySize: 24

lld/test/ELF/gnu-ifunc-i386.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=i686-pc-linux %s -o %t.o
 // RUN: ld.lld -static %t.o -o %tout
 // RUN: llvm-objdump -d --no-show-raw-insn %tout | FileCheck %s --check-prefix=DISASM
 // RUN: llvm-readobj -r --symbols --sections %tout | FileCheck %s
 
 // CHECK:      Sections [
 // CHECK:       Section {
 // CHECK:       Index: 1
 // CHECK-NEXT:  Name: .rel.dyn
 // CHECK-NEXT:  Type: SHT_REL
 // CHECK-NEXT:  Flags [
 // CHECK-NEXT:    SHF_ALLOC
+// CHECK-NEXT:    SHF_INFO_LINK
 // CHECK-NEXT:  ]
 // CHECK-NEXT:  Address: [[RELA:.*]]
 // CHECK-NEXT:  Offset: 0xD4
 // CHECK-NEXT:  Size: 16
 // CHECK-NEXT:  Link: 0
 // CHECK-NEXT:  Info: 4
 // CHECK-NEXT:  AddressAlignment: 4
 // CHECK-NEXT:  EntrySize: 8

lld/test/ELF/gnu-ifunc.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 // RUN: ld.lld -static %t.o -o %tout
 // RUN: llvm-objdump -d %tout | FileCheck %s --check-prefix=DISASM
 // RUN: llvm-readobj -r --symbols --sections %tout | FileCheck %s
 
 // CHECK:      Sections [
 // CHECK:       Section {
 // CHECK:       Index: 1
 // CHECK-NEXT:  Name: .rela.dyn
 // CHECK-NEXT:  Type: SHT_RELA
 // CHECK-NEXT:  Flags [
 // CHECK-NEXT:    SHF_ALLOC
+// CHECK-NEXT:    SHF_INFO_LINK
 // CHECK-NEXT:  ]
 // CHECK-NEXT:  Address: [[RELA:.*]]
 // CHECK-NEXT:  Offset: 0x158
 // CHECK-NEXT:  Size: 48
 // CHECK-NEXT:  Link: 0
 // CHECK-NEXT:  Info: [[GOTPLT:.*]]
 // CHECK-NEXT:  AddressAlignment: 8
 // CHECK-NEXT:  EntrySize: 24

lld/test/ELF/x86-64-combined-dynrel.s

 _start:
  call func@plt
  movq func@GOTPCREL (%rip), %rax
 
 # CHECK:     Name: .rela.dyn
 # CHECK-NEXT:         Type: SHT_RELA
 # CHECK-NEXT:     Flags [
 # CHECK-NEXT:       SHF_ALLOC
+# CHECK-NEXT:       SHF_INFO_LINK
 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Address:
 # CHECK-NEXT:     Offset:
 # CHECK-NEXT:     Size: 48
 
 # CHECK:  0x0000000000000008 RELASZ               48
 # CHECK:  0x0000000000000002 PLTRELSZ             24